Gen. John Campbell testified Tuesday that the US bombing that killed 22 at a Doctors Without Borders hospital in Kunduz, Afghanistan, was not intentional, shifting the Pentagon's initial account of 'collateral damage.'

Members of CodePink protest the deadly American attack on a hospital in northern Afghanistan as U.S. Forces-Afghanistan Resolute Support Mission Commander Gen. John Campbell, right, testifies on Capitol Hill in Washington, Tuesday, Oct. 6, 2015, before the Senate Armed Services Committee hearing on the Situation in Afghanistan.

The American airstrike that killed at least 22 people at an Afghan Doctors Without Borders hospital in the Afghan city of Kunduz was a mistake, according to the top commander of US and allied forces in Afghanistan.

"The decision to provide aerial fires was a US decision, made within the US chain of command," US Army Gen. John Campbell told the Senate Armed Services Committee on Tuesday. "The hospital was mistakenly struck. We would never intentionally target a protected medical facility."

He promised a "thorough, objective and transparent" investigation into the incident. He also said Afghan forces had requested air support from the United States, having been in combat with Taliban forces in Kunduz.

"Afghan forces on the ground requested air support from our forces there on the ground," he said, "but as I said in my opening statement, even though the Afghans request that support, it still has to go through a rigorous US procedure to enable fires to go on the ground.”

The Saturday bombings went on for than 30 minutes and the hospital burned for hours afterwards. Of those killed, 12 were staff members and 10 were patients, three of whom were children.

Doctors Without Borders, known internationally as Médecins Sans Frontières (MSF), has demanded an independent investigation of the incident.

"[The US] description of the attack keeps changing – from collateral damage, to a tragic incident, to now attempting to pass responsibility to the Afghanistan government," MSF General Director Christopher Stokes said in a statement. "With such constant discrepancies in the US and Afghan accounts of what happened, the need for a full transparent independent investigation is ever more critical."

According to local authorities, Taliban militants were inside the hospital and had been targeting Afghan forces. But MSF denies this. The international medical charity reported, "Not a single member of our staff reported any fighting inside the MSF hospital compound prior to the US airstrike on Saturday morning."

The actual target of the airstrike remains unclear, as initial reports said that the airstrike aimed for individuals "in the vicinity" of the hospital. It’s protocol for US forces to verify a target before firing, and MSF said that the organization had informed both the US and Afghan authorities of its GPS coordinates. The Pentagon has changed its narrative of the incident since Saturday.

War protesters with red paint on their faces and hands attended the Tuesday hearing. At one point, a woman shouted "Bombing hospitals is a war crime! Stop the bombing now!" and was escorted out of the room, The Associated Press reports.

Campbell repeatedly emphasized that the US would never knowingly target a hospital.

"We continue to make extraordinary efforts to protect civilians. No military in history has done more to avoid harming innocents," he told the committee. "We've readily assumed greater risks to our own forces in order to protect noncombatants."

In his address before the Senate committee, Campbell also called for President Obama to revise his plan to diminish the US military presence in Afghanistan at the end of 2016, saying that counterterrorism missions and the ability to train Afghan forces would be greatly limited if the number of US troops decreased to 1,000 by the end of next year.